Deactivation Of External Acid Sites Research Articles

Lanthanide oxide modified H-Mordenites: Deactivation of external acid sites in the isopropylation of naphthalene

The external surface of the H-Mordenite (MOR) was modified with lanthanide oxides such as CeO2, La2O3, Pr2O3, Sm2O3, Dy2O3, and Yb2O3, to investigate the deactivation of external acid sites. It is confirmed that the modification with the oxides prevents the isomerization of 2,6-diisopropylnaphthalene (DIPN) during the isopropylation of naphthalene (NP). The oxides accumulated on the MOR surface deactivated the acid sites with keeping catalytic activities of the isopropylation when their loadings are in the range of 5–10 wt%; however, the differences in the activities were observed when the loading was increased to 20–30 wt%: CeO2 gave a high activity with a high selectivity for 2,6-DIPN, and Sm2O3 maintained moderate activities, whereas the other oxides lost the activities. The differences of the modified MORs are ascribed to the morphologies of mesopores formed by the accumulation of the oxides. CeO2 is accumulated as segregated nano-sized crystallites, and the pore-entrances of MOR are opened through their mesopores for the access of NP and its products, and the isopropylation occurs in the channels with a high selectivity for 2,6-DIPN without a significant loss of the activities. However, La2O3 is accumulated as amorphous glassy oxide with a low surface area, leading to the loss of the activities by disturbing the access of NP to MOR pores. Further, amorphous glassy Sm2O3 also supports the selective formation of 2,6-DIPN as the mesopores allow the access of NP to MOR channels.

Activation of Mo/HZSM-5 for methane aromatization

The effect of Mo/HZSM-5 pretreatment at 973 K in inert (He), oxidizing (artificial air), and carburizing (CH4/He mixture) atmospheres on its performance in non-oxidative methane dehydroaromatization (MDA) was investigated. The effect of post-synthesis silylation on deactivation of external acid sites was also studied. Precarburization resulted in increased aromatic selectivity and improved catalyst stability. The benzene selectivity was the highest for the silylated Mo/HZSM-5 catalyst (benzene + naphthalene selectivity after 1 h on stream was close to 100%). The deactivation of precarburized zeolites was less pronounced than that of zeolites heated in air or He. During heating in air or He, larger fractions of the molybdenum oxide species diffused into the micropores than during heating in methane. Carburization of the molybdenum oxide species in the micropores during MDA resulted in the formation of molybdenum carbide particles, and these contributed to pore blocking, making the Brønsted acid sites inaccessible. The formation of molybdenum carbides during heating in methane resulted in a less mobile Mo phase. It is argued that the presence of molybdenum carbide particles in the micropores contributes to rapid catalyst deactivation, in addition to the formation of hard coke on the external surface.

Deactivation of External Acid Sites of H-Mordenite by Modification with Lanthanide Oxides for the Isopropylation of Biphenyl and the Cracking of 1,3,5-Triisopropylbenzene and Cumene

The modification of H-mordenite (MOR) with lanthanide oxides La2O3, CeO2, Pr2O3, Sm2O3, Dy2O3, and Yb2O3 was examined for the deactivation of external acid sites and confirmed in the cracking of 1,3,5-triisopropylbenzene (TIPB) and cumene (IPB) and in the isopropylation of biphenyl (BP). The cracking of TIPB, which cannot enter the pores of MOR, shows that external acid sites were effectively deactivated by the modification of MOR with the lanthanide oxides in small amounts. Only the cracking of IPB over CeO2-modified MOR exhibited excellent catalytic activities, even at a 30 wt % metal loading, whereas the activities of other lanthanide oxide-modified MORs rapidly decreased as the loadings were increased because pore entrances became choked. The isomerization of 4,4′-diisopropylbiphenyl (4,4′-DIPB) during the isopropylation of BP at high temperatures such as 300 °C was also effectively prevented by the modification of MOR with the lanthanide oxides. Particularly, CeO2-modified MOR remained highly active even at a 30 wt % loading. Other lanthanide oxides can deactivate the isomerization of 4,4′-DIPB at 5–10 wt % loadings without significant loss of the activities at 300 °C, while the activity was rapidly lost as the loading amount was increased. The physicochemical properties of lanthanide oxide-modified MORs indicate that the lanthanide oxides modify the surface properties of MOR. The amounts of N2, o-xylene, and NH3 adsorbed on MORs mostly remained high after CeO2 modification; however, they rapidly decreased when loadings of the other oxides increased. These results show that CeO2 remains the open pores at high loadings; however, the other oxides reduce the size of pore entrances as the loading is increased.

Shape-selective alkylation and related reactions of mononuclear aromatic hydrocarbons over H-ZSM-5 zeolites modified with lanthanum and cerium oxides

The La 2O 3 and CeO 2 modifications of H-ZSM-5 zeolites were examined to improve the shape-selectivity in the alkylation of mononuclear aromatic hydrocarbons and related reactions. The selectivities of p-diethylbenzene ( p-DEB) among diethylbenzene isomers were improved without significant loss of catalytic activity by the modification with these oxides in the ethylation of ethylbenzene (EB). The La 2O 3 modification enhanced the selectivities of p-DEB more effectively than the CeO 2 modification. The improvement of the selectivities over these oxides is due to the prevention of the isomerization of p-DEB at external acid sites. The para-selectivity for La 2O 3-modified H-ZSM-5 zeolites is improved due to “product selectivity” resulting from the preferential diffusion of p-DEB by the adjustment of pore entrance as well as by the deactivation of external acid sites. The enhancement of the formation of less bulky xylene isomer was also observed in the disproportionation of toluene and the isomerization of xylene isomers over La 2O 3- and CeO 2-modified H-ZSM-5 zeolites.

Deactivation of external acid sites of H-mordenite by silica-modification in the isopropylation of biphenyl

Although extensive isomerization of 4,4'-diisopropylbiphenyl (4,4'-DIPB) occurred at 300oC over H-mordenite, high selectivity for 4,4'-DIPB among DIPB isomers was observed over silica-modified H-mordenite. This is due to the prevention of the isomerization of 4,4'-DIPB by the deactivation of external acid sites.

Zeolite Catalyzed Alkylation of Biphenyl. Where Does Shape-Selective Catalysis Occur?

Liquid-phase alkylation of biphenyl (BP) was studied over large-pore zeolites. Selective formation of the least bulky products, 4,4′-diisopropylbiphenyl (4,4′-DIPB) occurred only in the isopropylation of BP over one- or two-dimensional zeolites, H-mordenite (HM), ZSM-12, SSZ-24, SAPO-5, SSZ-31, and CIT-5. These shape-selective catalyses are ascribed to steric restriction of transition state and to easiness of the substrates to enter into the pores. HM gave the highest selectivity among them. The dealumination of HM enhanced catalytic activity and the selectivity for 4,4′-DIPB because of the decrease of coke deposition. Non-regioselective catalysis occurs on external acid sites over HM with the low SiO2/Al2O3 ratio because severe coke deposition deactivates the acid sites inside the pores by blocking pore openings. The selectivity of DIPB isomers was changed with propylene pressure and/or with reaction temperature. Selective formation of 4,4′-DIPB was observed at moderate temperatures such as 250 °C, whereas the decrease of the selectivity of 4,4′-DIPB occurred at higher temperatures as 300 °C. 4,4′-DIPB yielded selectively under high propylene pressure (<0.3 MPa) at 250 °C, while the selectivity of 4,4′-DIPB decreased under low propylene pressure as 0.2 MPa. However, 4,4′-DIPB was almost exclusive isomer in the encapsulated DIPB isomers inside the pores under every temperature and pressure. The decrease of the selectivity of 4,4′-DIPB is due to the isomerization of 4,4′-DIPB on the external acid sites. The deactivation of external acid sites of HM was examined by the modification with cerium and other rare earth metal oxide on HM. Selectivities of 4,4′-DIPB were improved over modified HM even at high temperatures because of the suppression of non-regioselective alkylation and isomerization at the external acid sites. The ethylation of BP to ethylbiphenyls (EBPs) and diethylbiphenyls (DEBPs) was non-regioselective. The ethylation of BP to EBPs was controlled kinetically. However, there was difference in reactivities of EBPs and DEBPs for their further ethylation. 4-EBP was ethylated preferentially among the isomers, although the formation of 4,4′-DEBP was less selective. 4-EBP and 4,4′-DEBP have the highest reactivities among EBPs and DEBPs for the ethylation to polyethylbiphenyls (PEBPs). These results show that the environments of HM pores are too loose for shape-selective formation of the least bulky isomers, 4-EBP and 4,4′-DEBP, in the ethylation of BP, and that HM pores have enough space for the further ethylation of 4,4′-DEBP.

Ceria-modification of H-mordenites: The deactivation of external acid sites in the isopropylation of biphenyl and the isomerization of 4,4′-diisopropylbiphenyl

Ceria-modification is an effective method for the deactivation of external acid sites of H-mordenite (HM). The selectivity for 4,4′-diisopropylbiphenyl (4,4′-DIPB) in the isopropylation of BP over dealuminated HM such as HM(128) decreased with raising reaction temperatures or with decreasing propylene pressures. The decrease of the selectivity during the isopropylation is due to the isomerization of 4,4′-DIPB. Ceria-modification of HM(128) was highly effective for the prevention of the isomerization. The selectivity for 4,4′-DIPB was also improved in the case of HM with low SiO 2/Al 2O 3 ratio such as HM(10). The enhancement of the selectivities of 4,4′-DIPB by ceria-modification is ascribed to the decrease of external acid sites which are active in non-regioselective alkylation and isomerization of products.

Shape-selective alkylation of biphenyl catalyzed by H-Mordenites

Liquid phase alkylation of biphenyl was studied over large pore zeolites. Selective formation of the narrowest products, 4,4′’-diisopropylbiphenyl (4,4′’-DIPB), occurred only over HM among the zeolites with twelvemembered pore openings. These shape-selective catalyses are ascribed to steric restriction of transition state and to entrance of bulky substrates into the pores. The dealumination of HM enhanced catalytic activity and the selectivity of 4,4′’-DIPB because of the decrease of coke-deposition, while the activity and the selectivity were low over HM with the low SiO2/Al2O3 ratio. Non-regioselective catalysis occurs on external acid sites because severe cokedeposition deactivates the acid sites inside the pores by blocking pore openings. The selectivity of DIPB isomers was changed with propylene pressure and/or with reaction temperature. 4,4′’-DIPB yielded selectively under high propylene pressure (<0.3 MPa) at 250 °C, while the selectivity of 4,4′’-DIPB decreased under such low propylene pressure as 0.2MPa. Selective formation of 4,4′’-DIPB was observed at moderate temperature such as 250 °C, whereas the decrease of the selectivity of 4,4′’-DIPB occurred at higher temperature as 300 °C. However, 4,4′’-DIPB was almost exclusively isomer in the encapsulated DIPB isomers inside the pores under every pressure and temperature. These decreases of the selectivity of 4,4′’-DIPB are due to the isomerization of 4,4′’-DIPB on the external acid sites. The deactivation of external acid sites of HM was examined to reduce non-regioselective alkylation and isomerization. External acid sites were deactivated by calcination after impregnation of cerium on HM without the decrease in pore radii. Selectivities of 4,4′’-DIPB were improved even at high temperatures in the isopropylation of biphenyl because of the suppression of non-regioselective alkylation and isomerization at the external acid sites. The selectivity of 4,4′’-diethylbiphenyl (4,4′’-DEBP) in the ethylation of biphenyl was much lower than that in the isopropylation. Among the DEBP isomers, 4,4′’-DEBP has the highest reactivity for the ethylation to polyethylbiphenyls inside the pores, whereas the isopropylation of 4,4′’-DIPB was negligibly low inside the pores. These differences are ascribed to the difference in steric restriction at the transition state composed of substrate, alkylating agent, and acid sites inside the pores.

Deactivation of External Acid Sites of H-Mordenite with Ceria Modification in the Isopropylation of Biphenyl

Abstract Cerium modification of H-mordenite was highly effective for the deactivation of acid sites on external surfaces in the isopropylation of biphenyl. The selectivity for 4,4′-diisopropylbiphenyl (4,4′-DIPB) over unmodified H-mordenite decreased with the increase of reaction temperature because of the isomerization of 4,4′-DIPB at the external acid sites. The isomerization was effectively prevented by the ceria modification of H-mordenites.

Cerium impregnated H-mordenite as a catalyst for shape-selective isopropylation of naphthalene. Selective deactivation of acid sites on the external surface

The impregnation of cerium is the effective method for the deactivation of external acid sites of H-mordenite. The selectivity of 2,6-DIPN in the isopropylation of naphthalene was enhanced by the impregnation with such a large amount as 30–50 wt.-% of cerium without significant decrease of catalytic activity. The highest selectivity of 2,6-DIPN was achieved up to 70% over a highly dealuminated H-mordenite, (HM(128); SiO 2/Al 2O 3= 128, with 30 wt.-% of cerium. The enhancement of the selectivity is ascribed to the deactivation of external acid sites judging from the activity of the cracking reaction of 1,3,5-triisopropylbenzene. The effective pore radius was not reduced by the modification. The ceria is highly dispersed only on the external surface of H-mordenite without any formation of new kinds of acid sites. The 129Xe NMR observation suggfests that cerium is not in the pores, but on the external surfaces. The deactivation of the external acid sites is a characteristic property for cerium. Lanthanum and neodymium inhibited catalytic activity of the isopropylation because the pores were narrowed by their impregnation. A possible reason of the deactivation is ascribed to the amphoteric property of ceria.

Selective synthesis of dimethylamine over small-pore zeolites: IV. Effects of SiO 2 and Al 2O 3 coatings

Al 2O 3 and SiO 2 coatings are effective in increasing dimethylamine (DMA) and decreasing trimethylamine (TMA) selectivities of small-pore zeolites HRHO and HZK-5 used as methylamine catalysts. The HRHO catalysts typically contain chabazite or chabazite and pollucite impurities. SiO 2 is more effective than Al 2O 3 for improving DMA selectivity. SiO 2 coatings from monosilicic acid (MSA) reduce dimethyl ether (DME) yields over shallow-bed nitrogen (SBN)-calcined HRHO, whereas Al 2O 3 coatings and SiO 2 coatings [from tetraethylorthosilicate (TEOS)]do not. Correlations between thickness of SiO 2 (TEOS or MSA) coatings and DMA selectivity as well as a decrease in the n-hexane rate of sorption suggest a physical hindrance to egress of TMA from RHO channels and cages to the product stream. Coating deep-bed calcined H-pollucite with either Al 2O 3 or SiO 2 from TEOS reduces activity and increases DMA selectivity through deactivation of nonselective surface sites. Improvement of DMA selectivity by coating DB-calcined HRHO catalysts occurs primarily from (i) HRHO port constriction and (ii) deactivation of external acid sites of HRHO and H-pollucite and secondarily from deactivation of H-chabazite. Improvement of DMA selectivity by coating SBN-calcined HRHO catalysts occurs primarily from (i) HRHO port constriction and (ii) further deactivation of external acid sites of HRHO and amorphous H-chabazite and secondarily from the deactivation of H-pollucite. SiO 2 coatings on HZK-5 increased DMA selectivity and decreased DME yields, but reduced activity.